1. Field of the Invention
This invention relates generally to a method for manufacturing a composite bicycle frame with high strength and light weight.
2. Description of the Prior Art
Conventional bicycle frames are principally made from metallic materials which are developed from iron to chromium molybdenum steel and aluminum alloy. The weight of the metallic bicycle frame is relatively heavy because the higher specific gravity of metallic materials. Accordingly, a bicycle frame made from non-metallic materials such as carbon fiber composite materials has been developed.
The performance characteristics of carbon fiber composite materials are light weight, high specific strength, high specific rigidity, etc. The carbon fiber composite materials are applied to bicycle frames to reduce the weight. For conventional bicycle frames, the lug type carbon fiber bicycle frames take a large number. Bicycle frames of this type are made from individual carbon fiber tubes which are joined together by adhering aluminum joints therebetween. However, such bicycle frames have a serious problem, that is, the stress transferred between joints and carbon fiber tubes is only achieved by adhesive therebetween so that the strength is relatively weak. Hence the qualities of the bicycle frames are determined by whether the joint conditions are strong enough to support the frames. Furthermore, it is easy to cause stress concentrations and discontinuity around lug-tube joint of the frame because of different rigidity of distinct materials. A monocoque type frame has been developed to overcome the preceding drawbacks, that is, the whole frame is made from composite materials with metallic parts embedded in suitable positions, if necessary. Some manufactured monocoque type frames have been disclosed in U.S. Pat. Nos. 4,850,607, 4,889,355 and 4,902,458. These frames may provide advantages over conventional bicycle frames, due in some instances to increased strength and a reduced number of major structural joints. However, these frames have the following drawbacks:
(a) The method of molding the frame structure employs a female molding unit having three mold pieces. The whole molding unit is too thick (about 20 to 25 cm) and has heavy weight. It is difficult to transfer heat from the molding unit to the frame such that the molding process is slow and the frame has a longer molding cycle resulting in reduced production. If the molding unit is made from metallic materials, the manufacture of the molding unit is expensive because the large-sized molding unit and processing machinery, and the precision requirement.
(b) The frame is integrally molded by three mold pieces. It is inconvenient to place the laid up prepreg in the molding cavities and to assemble the separate mold pieces together because of the large-sized mold pieces. The prepreg is easily pinched by the mold pieces, particularly, the intermediate rear mold piece inserted between the right and left stays of the rear triangle frame component makes it easy to pinch the prepreg when the rear mold piece is pushed in to fit with the other mold pieces.
(c) The chain stay portions and the seat stay portions are formed by laying up resin impregnated fabric around solid foamable cores without using the better way of inflation. The frame which is formed by inflatable bladders has the advantages of as low cost, high strength, and light weight. However, the frame which is formed by using the foamed cores will increase its weight about 50 to 100 g since they are not easy to be removed and have to be left in place. If the foamed cores are removed by dissolution in an solvent to decrease the weight of the frame, the foam cores must use thermoplastics and the Tg of soluble thermoplastic foam is usually lower than 110.degree. C. It should be noted that the molding temperature of composite materials is about 120.degree. to 150 .degree. C., the foam cores will shrink at this temperature range (above the Tg). The shrinking will affect the quality of the frame because the pressure exerted by the foamed cores is not sufficient for adequate compaction. Furthermore, to remove the foamed cores by dissolution in an solvent is time-consuming and costly, and the solvent may weaken the strength of composite materials and cause pollution problem.
(d) The tube portions are formed by overlapping the opposite parts of adjacent wall sections, wherein fibers at the overlapping edges are discontinuous (sheared between the overlapping edges). The frame may not have the necessary structural strength because the stress loadings are transferred from one wall section to the overlapping wall section just by the resin therebetween. In other words, the molded juncture is as strong as the resin between the overlapping wall sections. It is easy to rupture the frame at the overlapping edges after a certain period of using.